The present invention relates to an amplifier circuit of the type having a variable gain, particularly for use in an automatic, dynamic compander system employed in high fidelity sound reproduction equipment.
The invention is based on an amplifier circuit having a variable transmission factor, i.e., variable gain or attenuation factor, and having a lower limit frequency which is variable by means of a control voltage, and provided with a feedback path between the output of the amplifier and an input which inverts the output signals of the amplifier.
In order to be able to shift the above-mentioned lower limit frequency in a simple manner, the amplifier is designed so that the feedback path includes a parallel circuit of an electronically controllable resistor and a first frequency dependent impedance, a second frequency dependent impedance is connected between the inverting input and a point providing a reference potential, which is normally ground potential, and the control voltage is fed to the control input of the controllable resistor. A circuit of this type is disclosed in U.S. application Ser. No. 703,117, filed by Gerhard Dickopp and myself on July 6, 1976.
Such a shift of the lower limit frequency is required, for example, in order to eliminate certain drawbacks of earlier compander systems.
One such drawback is that, under certain operating conditions, a high amplitude low frequency sound signal is unable to cover high frequency interfering signals, such as noise, for example. If such a high amplitude low frequency signal appears, no noticeable compander effect will take place because of the high signal level. The noise can then be fully heard, for example. If, however, the amplitude of the low frequency signal drops, the compander effect begins. Thus, there occurs the further drawback that the noise is modulated in dependence on the level of the low frequency signal.
In the Dolby system disclosed in German Auslegeschrift (Published Application) No. 14 87 276, which is a compander system having a first signal path for transmitting the useful signal with linear amplification and full bandwidth and a second signal path which influences the amplification of the useful signal in a certain frequency range as well as an auxiliary circuit which combines the output signals of the two signal paths, the lower limit frequency of the certain frequency range is shifted toward higher frequency values as the amplitude of the useful signal increases. Such an amplitude dependent shift of the lower limit frequency of that frequency range in which there is dynamic regulation enables the above-mentioned difficulties to be substantially avoided.
When the above-described amplifier is used, it is advantageous to provide a control voltage generator which derives a control voltage from the useful signal to be processed and whose output is connected to the input of the controllable resistor.
Such an amplifier has the additional advantage that it can also be used as a filter whose lower limit frequency can be varied by means of the controllable resistor.